In recent years, the demands for telecommunications data transmission services have increased sharply. In order to meet these demands, attempts have been made to adapt the large existing telecommunications voice transmission facilities for data transmission. These attempts have been greatly helped by the fact that much of the voice network, especially recent and planned additions to that network, is capable of transmitting voice signals in a digital format.
Voice signals are normally transmitted in digital format over digital facilities at a 64 kilobit per second (kb) rate using digital transmission channels adapted for this bit rate. Data sources and destinations usually only require the use of much lower bit rate channels such as 2.4, 4.8, or 9.6 kb. Therefore, in order to make optimum use of the existing voice transmission facilities in an alternative mode for transmitting data, prior art systems have combined or multiplexed a number of lower bit rate or subrate data channels onto a single voice or voice equivalent channel. One prior art system, the Digital Data System described in The Bell System Technical Journal (BSTJ), V. 54, No. 5, May-June 1975, pp. 811-964, is a transmission system adapted to the digital transmission of data of different bit rates. In the Digital Data System, as one example, 20 subrate data channels operating at a rate of 2.4 kb can be multiplexed onto a single full-rate voice equivalent channel.
In the most common prior art digital transmission systems, digital signals from a particular source are first assembled into bytes, which are further assembled into frames. Single byte signals for a full-rate channel are generated 8000 times per second. The interbyte interval, the frame time, for a full-rate channel is 125 microseconds ##EQU1## The interbyte interval or partial superframe time for a subrate channel is longer; for example, the partial superframe time for a 2.4 kb subrate channel is 20 frame times or 2500 microseconds. Prior art time-division switches are not adapted to switch channels having different interbyte intervals efficiently.
In a communication system, in order to interconnect arbitrary terminals transmitting and receiving over subrate channels, it is necessary to individually switch the subrate channels conveyed by each full-rate channel. Prior art time-division switches which switch all signals at a standard full-rate, such as the 4ESS.TM. switch of the Western Electric Company, are not efficient when used for switching individual subrate channels to different destinations. When such switches are used for subrate channel traffic, the subrate channels must be demultiplexed and each channel brought up to a full-rate channel before the subrate channel signals can be switched. This is very costly when the fraction of subrate channel traffic is substantial.
In another prior art system, subrate channel switching is accomplished by connecting each subrate channel directly to a time-division switch. This is also very uneconomical since the cost of the multiplex facilities to assemble a number of subrate channels to a single switch input is much less than the cost of additional inputs connected directly to a time-division switch.